Planar waveguide with patterned cladding and method for producing the same

ABSTRACT

Integrated optical waveguides and methods for the production thereof which have a patterned upper cladding with a defined opening to allow at least one side or at least one end of a light transmissive element to be air clad. The at least one side or at least one end is, for preference, a lens structure unitary with the waveguide, or a bend.

FIELD OF THE INVENTION

The invention relates to an integrated optical waveguide with apatterned upper cladding, and to methods for patterning the uppercladding.

BACKGROUND OF THE INVENTION

Integrated optical waveguides typically consist of a patterned, lightguiding core layer (of refractive index n₁) surrounded by a claddingmaterial (of refractive index n₂, where n₂<n₁) and mounted on amechanically robust substrate. These waveguides generally have flat endfaces, often produced by cutting the substrate and waveguide structurewith a dicing saw, followed by a polishing step to remove scatteringcentres. Light propagating along the waveguide is guided within the coreby the refractive index difference between core and cladding.

Referring to the drawings, FIGS. 1 a and 1 b show side and end views ofthe end face of a typical integrated optical waveguide 10 as known inthe art, comprising a substrate 11, a lower cladding layer 12, a lightguiding core 13 and an upper cladding layer 14. Depending on thematerial system, a variety of techniques are available for depositingthe lower cladding, core and upper cladding layers. These include flamehydrolysis or chemical vapour deposition (e.g. for glass), molecularbeam epitaxy (e.g. for semiconductors) and spin coating (e.g. forpolymers). The core layer can be patterned either by photolithographyand reactive ion etching (suitable for most materials) or byphotolithography and wet etching (e.g. for photo-patternable polymers).The refractive index of the lower 12 and upper 14 cladding layers needsto be less than that of the core 13, so that light is confined withinthe core. Often, the lower 12 and upper 14 cladding layers have the samerefractive index, so that the guided mode is symmetric, although this isnot necessary. If the substrate material 11 is transparent and hasrefractive index lower than the core material 13, the lower cladding 12may be omitted.

Typically, planar waveguides have a light transmissive elongated coreregion which is square or rectangular in cross section. The bottom faceis conventionally defined as that being adjacent or nearest thesubstrate. The top face is the face parallel to the bottom face butfurthest from the substrate. The sides are those faces which areperpendicular to the substrate.

In this integrated optical waveguide previously described in the art,the core is surrounded by cladding material, either the lower claddingor the upper cladding. However this need not necessarily be the case,and there are some applications where it is advantageous for at leastone portion of the core to be free of contact with cladding material onat least one side. Accordingly, one aspect of the present inventionconcerns integrated optical waveguides where the upper cladding layer ispatterned such that in at least one region, at least one side of thecore is free of contact with the upper cladding material.

Patterned upper claddings have been disclosed in U.S. Pat. No. 5,850,498and U.S. Pat. No. 6,555,288, for reducing the stress in a waveguidecore. In these disclosures, where the patterned upper cladding isdescribed as “conformal”, the upper cladding has a shape substantiallycongruent with the shape of the core, in other words the core isenclosed (except the bottom, which is in contact with the substrate orlower cladding material) with a thin layer of upper cladding material.This is distinct from the patterned upper cladding of the presentinvention, where the upper cladding layer is patterned such that in atleast one region, at least one side of the core is free of contact withthe upper cladding material.

One application where it is advantageous for at least one region of thecore to be free of contact with cladding material on at least one sideis an integrated optical waveguide with a unitary lens structure. Asmentioned above, light is guided along an integrated optical waveguideby the refractive index difference between the core and cladding layers.However when the light exits the core into free space (or air inpractice) it immediately diverges. This divergence occurs in twodimensions, parallel and perpendicular to the substrate. If a collimatedoutput beam is desired, some sort of positive (i.e. converging) lens isrequired. Equivalently, a converging lens is required to focus acollimated beam into an integrated optical waveguide.

One solution is to use a discrete element such as a ball lens or acylindrical gradient refractive index (GRIN) lens, however such lensesare difficult to handle because of their small size, require precisealignment in two dimensions, and introduce additional interfaces (withinherent reflection losses). It is preferable therefore to integrate thelens structure with the optical waveguide. Many types of integratedlenses have been proposed over the years, including Fresnel lenses (U.S.Pat. No. 4,367,916; U.S. Pat. No. 4,445,759) and Bragg lenses (U.S. Pat.No. 4,262,996; U.S. Pat. No. 4,440,468). These lenses provide focussingin one dimension only, in the plane of the lens structure (invariablyparallel to the substrate).

Another possibility is to fabricate a GRIN lens at the end face (U.S.Pat. No. 5,719,973). These lenses provide focussing in two dimensionsbut have cylindrical symmetry and as such are more suited to opticalfibres than integrated optical waveguides (which are typicallyrectangular in shape).

One method for integrating a lens structure with an optical waveguide isto produce a lens-shaped protrusion on the end face of the waveguide.This may be achieved by selectively etching the cladding to leave aprotruding core, then heating the waveguide material to its softeningpoint (e.g. with a CO₂ laser pulse) so that the angular protrusioncollapses into a rounded convex lens shape. Such a structure alsoprovides focussing in two dimensions.

FIGS. 2 a, 2 b and 2 c illustrate a method for fabricating a lens, asknown in the art, on the end face of an integrated optical waveguide, asdescribed in U.S. Pat. No. 5,432,877. According to this embodiment, FIG.2 a shows the substrate 11 (e.g. silicon), the lower cladding 12 andupper cladding 14 (both comprising silica doped with boron andphosphorus) and the core 13 (comprising silica doped with boron,phosphorus and germanium). The end face of the waveguide 10 is etched ina buffered hydrofluoric acid solution, which preferentially etches thecladding layers, to leave a protrusion 20 of core material. Finally, theetched waveguide is heated to approximately 1000° C. to soften the core;surface tension then shapes the protruding core to produce asubstantially cone-shaped lens 21.

Such chemical etching techniques have been demonstrated for silicaglass-based waveguides in U.S. Pat. No. 5,432,877 and U.S. Pat. No.6,341,189. They are, however, limited in their applicability, relying ondifferential etch rates between the cladding (e.g. silica) and the core(e.g. germanium-doped silica). For example, selective chemical etchinggenerally cannot be used for polymer-based integrated opticalwaveguides. Furthermore, the thermal rounding process can only be usedif the core material has a softening point, which excludesnon-thermoplastic polymers and crystalline materials such as silicon andother semiconductors. Also, the etching and softening processes must beprecisely controlled if the desired lens shape is to be obtained.

A further disadvantage with chemical etching-based techniques is thatthe lens structures can only be prepared after the optical waveguidecircuit chips have been diced (cutting or breaking into individualchips). While many chips can be collected and etched at the same time,this still requires careful handling and extra process steps.

The present invention concerns a method for fabricating an integratedoptical waveguide with a unitary lens structure and with a patternedupper cladding that avoids some or all of the abovementioneddisadvantages of the prior art. The unitary lens comprises a curvedsurface through which light is launched into free space. Since thiscurved surface must have an air interface, any upper cladding must bepatterned such that at least this curved surface is free of contact withcladding material.

The unitary lens structures described in the present invention arecapable of focussing light in the dimension parallel to the substrate.If focussing in the perpendicular direction is desired, an external lenssuch as a transverse cylindrical lens could be used. This configurationis superior to other configurations known in the art that require anexternal ball or GRIN lens, because these require the external lens tobe accurately positioned in two dimensions. In contrast, an externaltransverse cylindrical lens would only need to be accurately positionedin the perpendicular direction. This is especially advantageous indevices with arrays of unitary lens structures, where one transversecylindrical lens could be used to provide perpendicular focussing for aplurality of array elements.

A second application where it is advantageous for at least one region ofthe core to be free of contact with cladding material on at least oneside is an integrated optical waveguide device where light is directedaround curves with small bend radii. This situation frequently arises inthe design of integrated optical waveguide devices, since the footprintof a device can be reduced (and therefore more devices fabricated persubstrate) by implementing tight bends. Without wishing to be bound bytheory, it is well known that introducing a bend into an opticalwaveguide perturbs the guided modes such that they tend to leak out theside of the bend, resulting in loss of optical power. For large bendradii (i.e. gradual bends) this loss is negligible, but as the bendradius is reduced, there comes a point where the loss becomesunacceptable. For a given bend radius, the loss depends on therefractive index difference between the core and cladding; if thisrefractive index difference is larger (i.e. the guided modes are moretightly bound), the loss is smaller. Bend-induced loss occurs for bothsingle mode and multimode waveguides. For the multimode case, the higherorder modes (which are less strongly guided) have higher bend loss (i.e.tend to be lost first).

Generally, the core-cladding refractive index difference is maximised(and hence bend loss minimised) when the core is surrounded by freespace (air in practical terms), i.e. the “cladding” has a refractiveindex of 1. For integrated optical waveguide devices, bends usuallyoccur in one plane only, parallel to the substrate. Since bend loss onlyoccurs in the plane of the bend (i.e. light leaks out through the sidewalls), only the side walls need to be “air clad”. In particular, onlythat side wall on the outside of the bend needs to be air clad.Referring now to FIGS. 1 a and 1 b: if an integrated optical waveguidedevice has tight bends, it would be advantageous for the upper cladding14 to be omitted. The core 13 would then be in contact with claddingmaterial (the lower cladding 12) only on the bottom, which in terms ofbend loss is unimportant.

A disadvantage with omitting the upper cladding in planar waveguides isthat the mechanical strength of the structure may be insufficient, i.e.the structure cannot be processed and handled using standard techniques.For example, dicing with a high speed saw could dislodge the core fromthe lower cladding. Also, bare core structures are extremely vulnerableto mechanical damage or to the formation of scattering centres byextraneous dust. For this reason, in integrated waveguide devices withtight bends, it is advantageous to pattern the upper cladding such thatupper cladding material is present everywhere except in the regionssurrounding the tight bends.

Similarly, in integrated waveguide devices with unitary lens structures,it is advantageous to pattern the upper cladding such that uppercladding material is present everywhere except in the regionssurrounding the unitary lens structures.

SUMMARY OF THE INVENTION

The first aspect of the present invention concerns integrated opticalwaveguides where the upper cladding is patterned such that the core isfree of contact with cladding material in at least one region, on atleast one side. The second aspect of the present invention describes amethod for patterning an upper cladding layer.

As mentioned above, planar waveguides have a light transmissiveelongated core region which is square or rectangular in cross section,with top and bottom faces and two sides. Such planar waveguides willalso have two ends. As these ends are perpendicular to the substrate,they may also be regarded for the purposes of the present invention asbeing sides.

According to a first aspect the invention provides a method forproducing an integrated optical waveguide with a patterned uppercladding comprising the steps of:

-   a) depositing a core layer onto a substrate, optionally with a lower    cladding layer therebetween;-   b) patterning the core layer to provide a light transmissive    element;-   c) depositing an upper cladding layer onto the light transmissive    element; and-   d) patterning the upper cladding to provide at least one region in    which the light transmissive element is air clad.

According to a second aspect the invention provides an integratedoptical waveguide with patterned upper cladding comprising: a substrate;an optional lower cladding layer; a light transmissive element; and apatterned upper cladding having at least one air clad region.

Patterning the core layer will generally result in an uncovered portionof the substrate or lower cladding layer (where used) being present. Itwill be understood that in these circumstances, this uncovered portionmay be recoated wholly or partially with the upper cladding when this isapplied. Patterning of the upper cladding may or may not result in someportions of the substrate or lower cladding layer being uncovered.

Also, while the present invention discloses that certain side or endregions of the light transmissive portion be left unclad, those skilledin the art will understand that apart from the functional regions ofinterest, the light transmissive core is best clad as much as possibleto avoid mechanical damage and scattering losses.

In one alternative embodiment, the light transmissive element comprisesa waveguide and lens as a unitary body. Preferably, the lens has acurved surface, and the step of patterning the upper cladding is suchthat the curved surface is free of contact with the upper claddingmaterial.

Alternatively, the light transmissive element comprises a waveguide witha bend. Preferably, the step of patterning the upper cladding is suchthat in the region of the bend, the waveguide is free from contact withthe upper cladding material. More preferably the waveguide is free fromcontact with the upper cladding material on the side corresponding tothe outside of the bend.

Preferably, the upper cladding layer comprises a polymeric material,more preferably a thermally curable polymer. It is thus preferred thatthe upper cladding layer is patterned by selectively curing it with apatterned heat source and the uncured material dissolved with a solvent,whereby the cured material is insoluble in the solvent.

Alternatively, the upper cladding layer comprises a polymer curable byactinic radiation, preferably ultraviolet light. It is thus preferredthat the upper cladding layer is patterned by selectively curing it witha patterned source of ultraviolet light and the uncured materialdissolved with a solvent, whereby the cured material is insoluble in thesolvent.

It is generally preferred that the polymer is a siloxane polymer, and itis also preferred that the substrate comprises silicon, silica, glass orpolymer. Non-limiting examples of polymer substrates include: acrylic,Perspex, polymethylmethacrylate, polycarbonate, polyester,polyethyleneterephthalate and PET. Preferably, the lower cladding andlight transmissive element comprise materials selected from polymericmaterials, glass and semiconductors.

More preferably, the lower cladding and light transmissive elementcomprise a polymer, preferably a siloxane polymer, curable by actinicradiation, most preferably ultraviolet light.

According to a third aspect the invention provides a method offabricating an optical waveguide device with a patterned upper cladding,comprising the steps of:

-   a) forming a patterned blocking layer opaque to a predetermined    wavelength on a portion of a substrate transparent to the    predetermined wavelength;-   b) depositing a core layer on said patterned blocking layer and/or    on an uncovered portion of the substrate;-   c) patterning the core layer from above to provide a light    transmissive element;-   d) depositing an upper cladding layer, which comprises a material    curable by exposure to light of the predetermined wavelength on the    light transmissive element, and/or on the patterned blocking layer    and/or on an uncovered portion of the substrate;-   e) irradiating said upper cladding layer from below with light of    the predetermined wavelength, to cure those portions of said upper    cladding layer not positioned above said patterned blocking layer;    and-   f) removing non-cured portions of said upper cladding layer.

According to a fourth aspect the invention provides a method offabricating an optical waveguide device with a patterned upper cladding,comprising the steps of:

-   a) forming a patterned blocking layer opaque to a predetermined    wavelength on a portion of a substrate transparent to the    predetermined wavelength;-   b) depositing a lower cladding layer on said blocking layer and/or    on an uncovered portion of said substrate;-   c) depositing a core layer on said lower cladding layer;-   d) patterning the core layer from above to provide a light    transmissive element;-   e) depositing an upper cladding layer on said core layer and/or on    an uncovered portion of said lower cladding, which comprises a    material curable by exposure to light of the predetermined    wavelength;-   f) irradiating said upper cladding layer from below with light of    the predetermined wavelength, to cure those portions of said upper    cladding not positioned above said patterned blocking layer; and-   g) removing non-cured portions of said upper cladding layer.

Preferably the patterned blocking layer is formed by screen printing. Itpreferably comprises a compound that absorbs light of the predeterminedwavelength.

Preferably the upper cladding layer comprises a polymer curable byexposure to light of the predetermined wavelength, which is preferablyin the ultraviolet region. Preferably the polymer is a siloxane polymer.

In some embodiments, it is preferred that the patterned blocking layercomprises a pattern of scattering surfaces, such as may be formed bymechanical abrasion or chemical etching, for example, wherein thescattering surfaces scatter light of the predetermined wavelength,effectively blocking transmission of said light.

It will be appreciated that when the core layer is patterned onto asubstrate, an uncovered portion of substrate will usually result. Someor all of the patterned blocking layer may also be uncovered. Thesubstrate and or blocking layer will usually be recoated with the uppercladding layer. Following photolithography with curing light passingthrough the substrate, the portion above the blocking layer is usuallywashed away. Other regions may be masked, for example by mechanicalmeans, to leave substrate or coating layer uncovered.

It will also be appreciated that when the core layer is patterned onto alower cladding layer, an uncovered portion of lower cladding layer willusually result. Some or all of the patterned blocking layer may also beuncovered as well as some or all of the lower cladding. The substrate,lower cladding layer and or blocking layer will usually be recoated withthe upper cladding layer. Following photolithography with curing lightpassing through the substrate, the portion above the blocking layer isusually washed away. Other regions may be masked, for example bymechanical means, to leave substrate, lower cladding or coating layeruncovered.

According to a fifth aspect the invention provides a method offabricating an optical waveguide device with a patterned upper cladding,comprising the steps of:

-   a) depositing a lower cladding layer on a substrate transparent to    light of a predetermined wavelength;-   b) forming a patterned blocking layer opaque to light having the    predetermined wavelength on said lower cladding layer;-   c) depositing a core layer on said blocking layer and/or on an    uncovered portion of the lower cladding layer;-   d) patterning the core layer from above to provide a light    transmissive element;-   e) depositing an upper cladding layer, on said light transmissive    element and/or on said blocking layer and/or on said lower cladding    layer, which comprises a material curable by exposure to light of    the predetermined wavelength;-   f) irradiating said upper cladding layer from below with light of    the predetermined wavelength, to cure those portions of said upper    cladding not positioned above said patterned blocking layer; and-   g) removing non-cured portions of said upper cladding layer.

In one alternative preferred embodiment, the invention provides a methodfurther comprising the steps of:

-   i) forming a lift-off layer after forming the patterned blocking    layer and before depositing the lower cladding layer; and-   ii) removing the lift-off layer after removal of the non-cured    portions of said upper cladding layer, to separate the lower    cladding layer, light transmissive element and patterned upper    cladding from the substrate.

In yet another alternative preferred embodiment, the invention providesa method further comprising the steps of:

-   i) forming a lift-off layer on the substrate before depositing the    lower cladding layer; and-   ii) removing the lift-off layer after removal of the non-cured    portions of said upper cladding layer, to separate the lower    cladding layer, patterned blocking layer, light transmissive element    and patterned upper cladding from the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b illustrate the side and end views of the end face of atypical integrated optical waveguide as known in the art.

FIGS. 2 a–c illustrate the steps involved in a selective chemicaletching method for fabricating a lens structure on the end face of anintegrated optical waveguide (as known in the art).

FIGS. 3 a and 3 b illustrate top and side views of a patterned waveguideand unitary lens structure with air cladding, and with a transversecylindrical lens to focus the light in the perpendicular direction.

FIGS. 4 a and 4 b illustrate why a patterned waveguide and unitary lensstructure cannot be protected with a conventional (i.e. non-patterned)upper cladding.

FIGS. 5 a and 5 b illustrate a possible but impractical method forfabricating a patterned waveguide and unitary lens structure with aconventional upper cladding.

FIGS. 6 a and 6 b illustrate one possible method for fabricating apatterned waveguide and unitary lens structure with a patterned uppercladding.

FIGS. 7 a and 7 b illustrate another possible method for fabricating apatterned waveguide and unitary lens structure with a patterned uppercladding.

FIG. 8 illustrates a top view of an integrated optical waveguide withtights bends and with the upper cladding omitted.

FIG. 9 illustrates a top view of an integrated optical waveguide withtights bends and with an upper cladding.

FIG. 10 illustrates a top view of an integrated optical waveguide withtights bends and with a patterned material/air upper cladding.

FIGS. 11 a–h illustrate a method for providing a waveguide structurewith a patterned upper cladding.

FIG. 12 illustrates the dimensions of a waveguide core with unitary lensstructure used to produce a collimated output.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The first aspect of the present invention concerns integrated opticalwaveguides where the upper cladding is patterned such that the core isfree of contact with cladding material in at least one region, on atleast one side. This aspect will be illustrated by two non-limitingexamples: integrated optical waveguides with unitary lens structures;and integrated optical waveguides with tight bends. This aspect is bestsuited to integrated optical waveguides comprising polymeric materials,and photo-patternable polymers in particular, but is generallyapplicable to waveguides comprising a wide range of materials.

Considering first the example of patterned upper claddings for unitarylens structures, this description will concentrate on lenses forlaunching a collimated beam into free space. It should be understoodhowever that this is a non-limiting case. The same unitary lensstructures can equally be used to focus a collimated beam into awaveguide. Patterned upper claddings for other unitary lens structures,for example lenses to focus light from a laser diode into a waveguide,are also within the scope of the invention.

A top view of an air clad waveguide core 13 with unitary patterned lensstructure 30 is shown in FIG. 3 a, with a side view shown in FIG. 3 b.Also shown are a lower cladding 12 and a substrate 11. The unitary lensstructure 30 comprises a taper region 31 and a curved surface 32. Lightguided along the core 13 diverges into the taper region 31 and iscollimated in the direction parallel to the substrate by refraction atthe curved surface 32. Because the lens is unitary with the waveguide,there is no loss or scattering as would occur if there were an interfacebetween the two portions. If it is required to collimate the beam in thedirection perpendicular to the substrate, a transverse cylindrical lens33 can be used to produce a beam 34 collimated in two directions. Thecore 13 and unitary lens structure 30 may comprise any optical materialthat can be patterned into the required shape. They may for examplecomprise germanium-doped silica deposited by flame hydrolysis andpatterned by photolithography and reactive ion etching (RIE). Morepreferably they comprise an optically transparent polymer (deposited byspin coating for example) that can be patterned by photolithographyfollowed by wet development in a suitable solvent. One advantage withthis structure, and the photolithographic method used to fabricate it,is that the patterning is precise, limited only by the resolution of themask used for the photolithography. In particular, the curvature of thelens can be designed specifically to produce a highly collimated outputbeam. Alternatively, the curvature of the lens can be designed toproduce a convergent beam, or to collect light emitted from a laserdiode. The curvature of the lens, illustrated as the curved surface 32,can have any shape suitable for producing the required collimated orconvergent beam. It may for example comprise a portion of a circle,ellipse, parabola or hyperbola, or any curve generated by a polynomialequation. It may also comprise a plurality of straight segments thatapproximate a curved surface. In practice, the digitisation generallyinvolved in fabricating the necessary mask for patterning the unitarylens structure means that the curved surface will be composed of aplurality of straight segments. A second advantage is that the lens canbe produced before the optical waveguide circuits are diced.

It is desirable to provide the unitary core and/or lens with an uppercladding to protect it from mechanical damage. However the lens needs tobe able to launch light directly into free space, so the curved surface32 cannot be covered by any cladding that might otherwise be used toprotect the structure. This situation is illustrated in FIGS. 4 a (topview) and 4 b (side view). In this case the curved surface 32 is unableto refract the emerging light into a collimated beam; instead the lightcontinues to diverge before being refracted into a highly divergentoutput 40 as it emerges from the upper cladding 14. Even if the lenswere designed to launch a collimated beam into the upper claddingmaterial, the output beam would be perturbed by any imperfections in theplanarisation of the upper cladding.

If an upper cladding is deposited but not patterned, the necessary airinterface of the curved surface can only be restored by cutting it toshape, as shown in FIGS. 5 a (top view) and 5 b (side view). Althoughthe resulting structure has good mechanical integrity, the cuttingprocess is impractical since curved surfaces cannot be produced byconventional dicing saw or wafer cleaving methods. Furthermore, theshape and quality of the lens obtained by the cutting process wouldgenerally be inferior to that obtained by the photolithographic processdescribed herein.

According to the present invention, the waveguide and unitary lensstructure can be protected and the lens/air interface preserved by usinga patterned upper cladding. One possible non-limiting configuration fora unitary lens structure with patterned upper cladding is shown in FIGS.6 a (top view) and 6 b (side view). As before, the core 13 and unitarylens structure 30 may comprise any optical material that can bepatterned into the required shape. Preferably they comprise an opticallytransparent polymer (deposited by spin coating for example) that can bepatterned by photolithography followed by wet development in a suitablesolvent. However any other optical material, such as glass or asemiconductor, could be used for the core and unitary lens structure,patterned using RIE for example.

After the core 13 and unitary lens structure 30 have been formed, aphoto-patternable polymer can be spin coated over them. By masking thesection of uncured polymer above the curved surface 32, the remainder ofthe polymer can be cured by exposure to actinic radiation in aphotolithographic process. The masked (i.e. uncured) region of polymercan then be dissolved away with a suitable solvent, for example acetoneor isopropanol, resulting in a patterned upper cladding 60. In thisconfiguration, the upper cladding edge 61 may be positioned at any pointback from the taper/curved surface 62. It may for example be positionedin the vicinity of the core/taper vertex 63.

Preferably, the polymer contains cross-linkable functions, such ascarbon-carbon double bonds or epoxide groups, that can be cross-linkedby exposure to actinic radiation, thereby rendering the materialinsoluble in the chosen solvent. Preferably, the actinic radiation is UVlight, although ionising radiation such as X-rays or an electron beammay also be suitable. Since it is not necessary to have high resolutionin this photolithographic process, thermal activation of thecross-linking process, for example via a masked CO₂ laser, may also beapplicable.

A photo-patternable polymer is particularly preferred for the patternedupper cladding because of the ease and mild conditions (e.g. UV exposurefollowed by solvent development) by which it can be patterned. Of thevarious optical materials that could be used for the underlyingstructures (core, unitary lens structure and lower cladding), almost all(including glass, semiconductors and cross-linked polymers) will beundamaged by the low levels of UV light used for photolithography, or bythe solvent used for development. This may not be the case with otherpatterning techniques, such as RIE. In this instance, one would have touse an upper cladding material that was etched much more rapidly thanthe other materials. Alternatively one could employ etch stop barrierlayers, but at the cost of many additional process steps.

One particularly suitable material for a patterned upper cladding is aUV curable siloxane polymer, synthesised for example by a condensationreaction as disclosed in the U.S. Pat. No. 6,818,721. Siloxane polymershave excellent adhesion to a variety of materials, and are well suitedas a general purpose upper cladding. A photoinitiator or thermalinitiator may be added to increase the rate of curing. Examples ofcommercially available photoinitiators include1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 184),2-methyl-1[4-methylthio)phenyl]-2-morpholinopropan-1one (Irgacure 907),2,2-dimethoxy-1,2-diphenylethan-1-one (Irgacure 651),2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (Irgacure369), 4-(dimethylamino)benzophenone,2-hydroxy-2-methyl-1-phenyl-propan-1-one (Darocur 1173), benzophenone(Darocur BP),1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one(Irgacure 2959), 4,4′-bis(diethylamino) benzophenone (DEAB),2-chiorothioxanthone, 2-methyithioxanthone, 2-isopropylthioxanthone,benzoin and 4,4′-dimethoxybenzoin. For curing with visible light, theinitiator may for example be camphorquinone. A mixture of two or morephotoinitiators may also be used. For example, Irgacure 1000 is amixture of 80% Darocur 1173 and 20% Irgacure 184. For thermal curing,organic peroxides in the form of peroxides (e.g. dibenzoyl peroxide),peroxydicarbonates, peresters (t-butyl perbenzoate), perketals,hydroperoxides, as well as AIBN (azobisisobutyronitrile), may be used asinitiators.

The patterning technique preferred in the present invention, that ofphotolithographic patterning and wet development of polymers can thus beseen to be different in principle from the chemical etching techniquespreviously described in the art. Both involve selective removal of aportion of material, however in the case of chemical etching, theselectivity is based on a chemical reaction (different compositions havediffering reaction rates with the etchant), whereas in thephotolithographic/wet development method preferred for the presentinvention, selectivity is based on a physical process (polymers withdifferent degrees of cross-linking have differing solubilities).

Another non-limiting configuration for a unitary lens structure withpatterned upper cladding is shown in FIGS. 7 a (top view) and 7 b (sideview). In this configuration, the upper cladding 71 is patterned so asto cover the entire taper region 31 and maintain the air interface ofthe curved surface 32. The same materials and patterning techniques asdescribed above in relation to FIGS. 6 a and 6 b could be employed forthis configuration, and again a UV curable siloxane polymer isparticularly preferred as the upper cladding material. Selectivitybetween the core/lens and upper cladding materials is not required inthis configuration. Patterning could therefore be done by RIE or morepreferably by UV-induced cross-linking followed by solvent development.

Once the upper cladding has been patterned, the desired integratedoptical waveguide device may be obtained by dicing arbitrarily close tothe curved surface of the lens. The dicing direction will usually beperpendicular to the direction of propagation along the core, but otherdicing directions may be used depending on the device and application.The device may contain one or more arrays of unitary lens structures, inwhich case it is advantageous to position the lenses in each arrayparallel to each other for ease of dicing.

Considering now the example of integrated optical waveguides with tightbends, FIG. 8 illustrates the top view of an integrated opticalwaveguide 80 with tight bends 81 and with the upper cladding omitted 82(i.e. air clad). This configuration is known in the art and has theadvantage of low bend loss because the core/cladding refractive indexdifference is maximised in the plane of the bends, but the disadvantagesof low mechanical strength and susceptibility to scattering loss.

FIG. 9 illustrates the top view of an integrated optical waveguide withtight bends and with an upper cladding 90. This configuration is knownin the art and has the advantages of good mechanical strength and lowscattering loss but the disadvantage of high bend loss, since thedifference between the refractive indices of the core material and uppercladding material is insufficient to prevent excessive leakage of lightthrough the core/cladding interface on the outside of the bends.

FIG. 10 illustrates the top view of an integrated optical waveguide withtight bends and with a patterned material/air upper cladding 100, suchthat the upper cladding material is absent around those regions of thecore with tight bends. This inventive configuration combines theadvantages of good mechanical strength, low scattering loss and low bendloss. In FIG. 10, the upper cladding has been patterned such that inthose regions of the core with tight bends, the upper cladding is absentfrom the top surface and the side walls both on the inside and outsideof the bends. Note that it is only necessary for the upper cladding tobe absent from the side wall on the outside of the bend.

It should be noted that the inventive configuration shown in FIG. 10introduces transition losses as the guided modes propagate from amaterial clad region into an air clad region and back into a materialclad region. Those skilled in the art will appreciate that to make bestuse of the inventive configuration, one must balance this transitionloss against the minimisation of bend loss.

As for the unitary lens example, the core and upper cladding layers maycomprise any optical material that can be patterned into the requiredshape. Preferably they each comprise an optically transparent polymer(deposited by spin coating for example) that can be patterned byphotolithography followed by wet development in a suitable solvent.

For the “conformal” upper claddings disclosed in U.S. Pat. No.6,555,288, FIGS. 7A and 7B (of that disclosure) show that the uppercladding is patterned by a conventional photolithographic/wetdevelopment process (i.e. exposure from above to actinic radiationthrough a mask, followed by solvent development), in the same manner asthe core (as shown in FIGS. 4 and 5 of that disclosure). In this methodpreviously described in the art, the patterned exposure (i.e. actinicradiation through a mask) can be performed with a variety of imagingtools, for example a mask aligner, scanning projection aligner orstepper. Either way, the method requires an imaging tool to be usedtwice, once for the core and once for the upper cladding. The sameimaging tool could be used for both exposures, in which case the timetaken to produce the optical device is increased, or two imaging toolscould be used, with greater plant expense. In general, however, an uppercladding does not need to be patterned with the same degree of spatialaccuracy as a waveguide core. For example, the positioning of the uppercladding with respect to the core in FIG. 7B of U.S. Pat. No. 6,555,288,the positioning of the upper cladding with respect to the unitary lensstructure in FIG. 6, or the positioning of the upper cladding withrespect to the bends in FIG. 10, is not critical. Note that this is notthe case with the unitary lens structure of FIG. 7 of the presentapplication, where the patterned upper cladding has to be in perfectregistration with the curved surface. For this reason, the configurationshown in FIG. 6 is preferred for the unitary lens structure withpatterned top cladding.

In an alternative to the method previously described in the art, theupper cladding could be patterned by exposure to actinic radiation frombeneath, through a transparent substrate. One possible means forperforming this patterning technique, using UV-curable polymermaterials, and which forms the second aspect of the present invention,is illustrated in FIGS. 11 a to 11 h. An upper cladding mask pattern 110is deposited, for example by screen printing, onto a substrate 111transparent to the UV light used for curing the upper cladding. Suitablesubstrate materials include quartz, fused silica, glass and polymer.Non-limiting examples of polymer substrates include: acrylic, Perspex,polymethylmethacrylate, polycarbonate, polyester,polyethyleneterephthalate and PET. A lower cladding layer 12 is spincoated onto the substrate and cured from above with UV light. A corelayer 112 is spin coated onto the cured lower cladding layer, patternedby exposure to UV light 113 from above through a mask 114, and developedwith a solvent to leave waveguide core 13. An upper cladding layer 115is spin coated over the waveguide core, patterned by exposure to UVlight 116 from below through the upper cladding mask pattern 110 anddeveloped with a solvent to leave patterned upper cladding 117. Thepatterned upper cladding 117 is an arbitrary structure used to exemplifythe inventive technique.

The inventive technique described above is non-limiting, and there areseveral possible variations. The upper cladding mask pattern 110 couldbe screen printed onto the lower cladding layer rather than onto thesubstrate. In this case the actinic radiation does not pass through thelower cladding layer, which is advantageous if sharp boundaries in thepatterned upper cladding are required (limited by diffraction). Inanother variation, since the substrate is transparent, the lowercladding layer 12 may be omitted (so long as the refractive index of thesubstrate is less than that of the core). In yet another variation, theupper cladding mask pattern 110 could be printed using photolithography,although this requires an imaging tool and obviates the primaryadvantage of the inventive technique.

The upper cladding mask pattern 110 can be composed of any material thatabsorbs the actinic radiation (usually UV light) used to pattern theupper cladding. It may for example be composed of a metal such asaluminium or a pigment that absorbs UV light. Preferably, it is composedof a pigment, dye or metallic paint that is suitable for screenprinting. More preferably, it is composed of a material that stronglyabsorbs UV light but not the light guided in the core layer (usually inthe near infrared). Such materials, such as compounds with conjugatedcarbon-carbon double bonds and/or aromatic rings, are common and wellknown in the art. This is a highly advantageous property since the uppercladding mask pattern 110 is left in situ.

In yet another variation, a reverse mask pattern could be screen printedonto the transparent substrate, and the uncoated surface regionsroughened for example by sandblasting or etching (e.g. with hydrofluoricacid if the substrate is fused silica). The reverse mask pattern couldthen be removed, e.g. with a solvent, to leave the desired uppercladding mask as a pattern of roughened features that will scatter (andeffectively block) the actinic radiation.

“Irradiation from below” schemes are known for patterning core layers(U.S. Pat. No. 5,352,566; U.S. Pat. No. 6,037,105; U.S. Pat. No.6,256,441), but not for patterning upper cladding layers. The schemesdisclosed in U.S. Pat. No. 6,037,105 and U.S. Pat. No. 6,256,441 arevirtually identical, involving upper cladding mask patterns deposited onthe lower cladding, for patterning a UV-curable core layer. In U.S. Pat.No. 6,037,105, the “irradiation from below” scheme is used to preventcontamination of the core layer caused by contact with a photomask,while in U.S. Pat. No. 6,256,441 it is used to provide highly accuratecore patterns. In contrast, the object of the inventive scheme forpatterning upper cladding layers from below is to reduce the number offabrication steps required, in particular those steps involving animaging tool. The “irradiation from below” scheme for core patterningdisclosed in U.S. Pat. No. 5,352,566 is distinct in many respects,involving positive photoresist layers and reactive ion etching steps.

In yet another possible variation of the inventive upper claddingpatterning technique, a lift-off layer can be inserted between thesubstrate and the lower cladding layer and/or the upper cladding maskpattern. After the core and upper cladding layers have been patterned,the lift-off layer can then be removed to separate the lower cladding,patterned core and upper cladding layers from the substrate. Thisvariation is particularly preferred if a flexible, all polymer waveguideassembly is required. Another advantage of this variation is that itenables the upper cladding mask pattern to be removed. The lowercladding may comprise a combination of an additional flexible polymersubstrate material and deposited optical quality cladding material, andthe upper cladding mask layer can be formed on any one of several layers(transparent substrate, lift-off layer, optional flexible polymersubstrate or lower cladding). A similar lift-off layer scheme isdisclosed in U.S. Pat. No. 6,256,441, but in this disclosure it is thecore layer that is patterned from below, not the upper cladding.

It should be noted that the inventive method for patterning an uppercladding by exposure from below is applicable to any patterned uppercladding, not merely to the inventive patterned upper claddings of thepresent invention, where the upper cladding layer is patterned such thatin at least one region, at least one side of the core is free of contactwith the upper cladding material. For example the inventive method maybe used to pattern the “conformal” upper claddings disclosed in U.S.Pat. No. 5,850,498 and U.S. Pat. No. 6,555,288.

EXAMPLE 1

Following the procedure disclosed in U.S. patent application Ser. No.10/308562, a lower refractive index polymer A was prepared with aviscosity of 2500 cP (at 20° C.) and a refractive index (measured at 20°C. on an Abbe refractometer with room light) of 1.483. A higherrefractive index polymer B was prepared with a viscosity of 2200 cP (at20° C.) and a refractive index of 1.509 (at 20° C.). A suitablephotoinitiator was added to both polymer A and polymer B.

Polymer A was spin coated onto a silicon wafer and cured with UV lightfrom a mercury lamp, to form a lower cladding layer 20 μm thick and witha refractive index of 1.478 (at 20° C. and 1550 nm). Polymer B was spincoated onto the lower cladding, and patterned with UV light through amask; the unexposed polymer B material was then dissolved in isopropanolto form a core and unitary lens structure as shown in FIG. 3 a. The corewas 8 μm wide and 15 μm high, and had a refractive index of 1.505 (at20° C. and 1550 nm). Finally, polymer A was spin coated and patternedwith UV light through a mask; the unexposed polymer A material was thendissolved in isopropanol to form a patterned upper cladding as shown inFIG. 6 a.

FIG. 12 illustrates the dimensions of one particular waveguide core withunitary lens structure 120. In this case, the width 121 of the waveguidecore is 8 μm, the length 122 of the taper section is 580 μm, the endwidth 123 of the taper section is 850 μm, and the curved surface 32 is aparabola defined such that it extends a distance 124 from the end of thetaper equal to 550 μm. For light of wavelength 850 nm guided in thecore, this unitary lens structure will produce a collimated output beamwith an approximate width of 850 μm in the direction parallel to thesubstrate.

EXAMPLE 2

Polymer A from Example 1 was spin coated onto a silicon wafer and curedwith UV light from a mercury lamp, to form a lower cladding layer 20 μmthick and with a refractive index of 1.478 (at 20° C. and 1550 nm).Polymer B from Example 1 was spin coated onto the lower cladding, andpatterned with UV light through a mask; the unexposed polymer B materialwas then dissolved in isopropanol to form a core with four tight bendsas shown in FIG. 8. The core was 8 μm wide and 15 μm high, and had arefractive index of 1.505 (at 20° C. and 1550 nm). Finally, polymer Awas spin coated over the core and patterned with UV light through amask; the unexposed polymer A material was then dissolved in isopropanolto form a patterned upper cladding as shown in FIG. 10.

EXAMPLE 3

Polymer A was meniscus coated onto an acrylic substrate (Perspex, orpolymethylmethacrylate), and cured with UV light from a mercury lamp, toform a lower cladding layer 10 μm thick and with a refractive index of1.478 (at 20° C. and 1550 nm). Polymer B was meniscus coated onto thelower cladding, and patterned with UV light using a scanning projectionaligner; the unexposed polymer B material was then dissolved inisopropanol to form a core and unitary lens structure as shown in FIG. 3a. The core was 8 μm wide and 20 μm high, and had a refractive index of1.505 (at 20° C. and 1550 nm). Finally, polymer A was meniscus coatedover the core and patterned with a scanning projection aligner; theunexposed polymer A material was then dissolved in isopropanol to form apatterned upper cladding as shown in FIG. 6 a.

EXAMPLE 4

A polyvinylalcohol (PVA) release layer was spin coated onto a glasswafer. A photocurable polymer, Norland NOA65, was spin coated and curedwith UV light from a mercury lamp to give a 100 μm thick layer. PolymerA was then spin coated and cured with UV light from a mercury lamp, toform a lower cladding layer 10 μm thick and with a refractive index of1.478 (at 20° C. and 1550 nm). Polymer B was spin coated onto the lowercladding, and patterned with UV light using a mask aligner; theunexposed polymer B material was then dissolved in isopropanol to form acore and unitary lens structure as shown in FIG. 3 a. The core was 8 μmwide and 10 μm high, and had a refractive index of 1.505 (at 20° C. and1550 nm). Polymer A was spin coated over the core and patterned with amask aligner; the unexposed polymer A material was then dissolved inisopropanol to form a patterned upper cladding as shown in FIG. 6 a. Thecoated wafer was then inserted into water to dissolve the PVA layer anda free-standing polymer film containing the waveguides with an uncoveredlens was released from the wafer. Excess polymer film was removed togive the desired waveguide on a free-standing plastic substrate.

EXAMPLE 5

A masking layer of UV-absorbing dye was screen printed onto a fusedsilica substrate, to cover those regions where the upper claddingmaterial is to be removed to form the patterned upper cladding shown inFIG. 6 a. As in Example 1, polymer A was spin coated and cured to form alower cladding layer, and polymer B was spin coated, patterned with UVlight and developed with isopropanol to form a core and lens structureas shown in FIG. 3 a. Polymer A was then spin coated and cured by UVlight from below, with the desired pattern transferred through theUV-absorbing dye masking layer. The unexposed polymer A material wasthen dissolved in isopropanol to form a patterned upper cladding asshown in FIG. 6 a.

EXAMPLE 6

A masking layer of UV-absorbing dye was screen printed onto an acrylicsubstrate (Perspex, or polymethylmethacrylate), to cover those regionswhere the upper cladding material is to be removed to form the patternedupper cladding shown in FIG. 6 a. Polymer A was meniscus coated onto thesubstrate and cured with UV light from a mercury lamp, to form a lowercladding layer 10 μm thick and with a refractive index of 1.478 (at 20°C. and 1550 nm). Polymer B was meniscus coated onto the lower cladding,and patterned with UV light using a scanning projection aligner; theunexposed polymer B material was then dissolved in isopropanol to form acore and unitary lens structure as shown in FIG. 3 a. The core was 8 μmwide and 15 μm high, and had a refractive index of 1.505 (at 20° C. and1550 nm). Polymer A was then spin coated and cured by UV light frombelow, with the desired pattern transferred through the UV-absorbing dyemasking layer. The unexposed polymer A material was then dissolved inisopropanol to form a patterned upper cladding as shown in FIG. 6 a.

1. An integrated optical waveguide comprising: a substrate; a lighttransmissive element comprising a waveguide and a lens as a unitarybody; an upper cladding patterned to have at least one region in whichthe light transmissive element is air clad; and wherein said lens has aface perpendicular to the substrate and focuses and collimates light ina plane parallel to the substrate and a lens face width at least 50%larger than the waveguide.
 2. An integrated optical waveguide accordingto claim 1 wherein the light transmissive element is air clad on atleast one side.
 3. An integrated optical waveguide according to claim 2,wherein the light transmissive element comprises a waveguide with abend.
 4. An integrated optical waveguide according to claim 3, whereinthe waveguide has an air clad surface in the region of the bend.
 5. Anintegrated optical waveguide according to claim 4, wherein the waveguidehas an air clad surface on the side corresponding to the outside of thebend.
 6. An integrated optical waveguide according to claim 1, whereinthe upper cladding is chosen from a group comprising an organosiliconcondensate polymer.
 7. An integrated optical waveguide according toclaim 1, wherein the substrate comprises silicon, quartz, fused silica,glass, or a polymeric material.
 8. An integrated optical waveguideaccording to claim 7, wherein the polymeric material comprises anacrylate, Perspex, polymethylmethacrylate, polycarbonate, polyester,polyethyleneterephthalate or PET.
 9. An integrated optical waveguideaccording to claim 1 wherein the light transmissive element comprisesmaterials selected from polymeric materials, glass and semiconductors.10. An integrated optical waveguide according to claim 1 including alower cladding layer between the substrate and the light transmissiveelement.
 11. An integrated optical waveguide according to claim 10wherein the lower cladding layer comprises materials selected frompolymeric materials, glass and semiconductors.
 12. An integrated opticalwaveguide comprising: a substrate; one or more light transmissiveelements each comprising a waveguide and a lens as a unitary body; andone or more cladding layers comprising at least one cladding layerpatterned to have at least one region with the cladding material removedfrom at least one region of the one or more light transmissive elements;wherein the lens has a face perpendicular to the substrate and a lensface width at least 50% larger than the waveguide and focuses andcollimates light in a plane parallel to the substrate.
 13. Theintegrated optical waveguide of claim 12 wherein at least one of saidone or more cladding layers is composed of an organosilicon condensatepolymer.
 14. The integrated optical waveguide of claim 12 wherein saidone or more light transmissive elements and at least one of said one ormore cladding layers are composed of materials chosen from a groupcomprising organosilicon condensate polymers, polymers, quartz, glassand semiconductors.
 15. The integrated optical waveguide of claim 12wherein said substrate is composed of materials chosen from a groupcomprising silicon, quartz, fused silica, glass, or a polymericmaterial.